/*
 * Copyright 2002-2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.accept;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

import org.springframework.http.HttpHeaders;
import org.springframework.http.InvalidMediaTypeException;
import org.springframework.http.MediaType;
import org.springframework.web.HttpMediaTypeNotAcceptableException;
import org.springframework.web.context.request.NativeWebRequest;

/**
 * A {@code ContentNegotiationStrategy} that checks the 'Accept' request header.
 *
 * @author Rossen Stoyanchev
 * @author Juergen Hoeller
 * @since 3.2
 */
public class HeaderContentNegotiationStrategy implements ContentNegotiationStrategy {

    /**
     * {@inheritDoc}
     *
     * @throws HttpMediaTypeNotAcceptableException if the 'Accept' header cannot be parsed
     */
    @Override
    public List<MediaType> resolveMediaTypes(NativeWebRequest request)
            throws HttpMediaTypeNotAcceptableException {

        String[] headerValueArray = request.getHeaderValues(HttpHeaders.ACCEPT);
        if (headerValueArray == null) {
            return Collections.emptyList();
        }

        List<String> headerValues = Arrays.asList(headerValueArray);
        try {
            List<MediaType> mediaTypes = MediaType.parseMediaTypes(headerValues);
            MediaType.sortBySpecificityAndQuality(mediaTypes);
            return mediaTypes;
        } catch (InvalidMediaTypeException ex) {
            throw new HttpMediaTypeNotAcceptableException(
                    "Could not parse 'Accept' header " + headerValues + ": " + ex.getMessage());
        }
    }

}
